This invention relates to a method for detecting phosphorus segregates, and more particularly, to such a method capable of rapidly and easily detecting the distribution of phosphorus in metallic materials such as continuously cast steel slabs and large-sized steel ingots.
Heretofore, segregation in large-sized steel ingots has been judged by sulfur printing. This method is by attaching photographic paper impregnated with aqueous sulfuric acid to a polished cross section of a large-sized steel ingot, thereby detecting hydrogen sulfide given off from segregated sulfur as stains on the photographic paper. This method has been widely used on the production line. Recently, however, steels subjected to low sulfide treatment and Ca treatment, such as steels resistant to hydrogen embrittlement cracking, have been put into practical use, and much progress has been made in the art to manufacture high purity steel and to minimize sulfur segregation in continuous castings. Such advanced steels having extremely low sulfur contents are difficult to detect solidification segregates by the conventional sulfur printing. It is thus desirable to detect phosphorus rather than sulfur for examining segregation.
Aside from the sulfur printing described above, a macroanalyzer is known as a device for examining the segregation of alloying elements. The macroanalyzer can quantitatively evaluate a planar section of a large-sized steel ingot by applying an electron beam to the section and detecting the spectrum of X-rays generated as in EPMA. However, this method is not applicable to a commercial production process because it uses an expensive device, the surface to be examined must be finished by emery paper of the order of #1,000, the measurement of a sample takes more than one hour, the configuration of a sample is limited, it cannot be applied to a wide section sample, and so on.
One known method of detecting phosphorus is the phosphorus printing reported by M. Niessner in 1932. This method is by attaching filter paper which has been immersed in liquid II shown below in Table 1 to a surface of steel to be examined for 3-5 minutes, removing the paper from the steel surface, and thereafter dipping the filter paper into liquid I for 3-5 minutes, thereby producing a printed image.
TABLE 1 ______________________________________ Liquid I Stannous chloride saturated solution 5 ml Concentrated hydrochloric acid 50 ml Water 100 ml Alum minor amount Liquid II Ammonium molybdate 5 g Water 100 ml Nitric acid (specific gravity 1.2) 35 ml ______________________________________
Since the specimen surface is maintained in contact with 1.8N nitric acid, the matrix is severely attacked and phosphorus is dissolved out there. When the removed test paper is dipped in liquid I, it turns blue over the entire surface. This method is only useful to estimate the amount of phosphorus in the matrix, but difficult to detect phosphorus segregates in commercial grade steels (see FIGS. 12 and 24).